Airway smooth muscle(ASM) mass and reactivity increase when the lungs are stressed by mechanical ventilation in diseases such as bronchopulmonary dysplasia (BPD)and Acute Respiratory Distress Syndrome (ARDS). To determine the effects of mechanical strain on ASM Cells isolated from other in vivo factors, we studied cultured ASM cells subjected to cyclic deformational strain. The cells exhibited increased proliferation, contractile protein content and actin filament content consistent with cell hypertrophy. Strain also increased tyrosine phosphorylation of two proteins which promote actin filament formation, paxillin and PP125FAK. Similar stress-induced cell hypertrophy and ultrastructural changes are thought to contribute to increased smooth muscle contractility and altered viscoelastic properties in several cardiopulmonary diseases. However, specific effects of these changes on cell contractility are difficult to study using tissue models because humoral mediators and connective tissue confound measurements. Therefore,we propose using single ASM cells in vitro to test the hypothesis that strain increases ASM cell contractility by increasing contractile protein content and actin filament formation. Accordingly, we will determine if: 1) strain-induced increases in contractile proteins are accompanied by increased force production, velocity of shortening or decreased cell relaxation. 2) strain-induced actin filament formation decreases cell compliance or maximal cell shortening. 3) strain-induced increases in protein tyrosine phosphorylation increase cell contractility or viscoelasticity of single cells. Strain-induced protein tyrosine phosphorylation will be inhibited by tyrosine kinase inhibitors and changes in single cell contractility and viscoelasticity assessed. These studies should elucidate mechanisms whereby mechanical stress alters ASM cell contractility. Examination of cultured cells will allow us to normalize variables not controlled in whole tissue models of ASM reactivity and to assign causative roles to individual components of the contractile apparatus.